Neutral density filters are well-known the art of theatrical lighting for the stage and related fields. Traditionally, colored filters are placed across the light beam.
A strip filter slide plate having the capability of transmitting light of varied intensities is positioned in operational association with an aperture through which a beam of light passes. The strip filter plate is moved across the beam of light so that the color saturation of the transmitted light is gradually increased or decreased in accordance with the position of a selected area of color density moved across the path of the liqht beam. Filters of the primary colors placed one behind the other are operated in series to create a desired color at a particular saturation level. Another configuration of a filter of a color changer is a rotatable disk having regions of variable intensity in the same manner as the strip filter plate type color changer.
The type of filters used in the art until recently has been absorption filters, which are generally made of glass. Spraying of color filter material on a translucent material is also done. A filter which passes a color of a particular wavelength, such as blue, for example, is moved across a beam aperture so that wavelengths of light not blue are absorbed. The density of the translucent colored material gradually changes so that the saturation level of the selected color transmitted can be gradually increased or decreased.
Recently, multilayered interference filters, also called dichroic filters, have been employed in color changing systems. The dichroic technique exploits subtractive chromatic characteristics of color-temperature-stabilized light sources. In brief, the dichroic technique comprises a multilayer interference filter construction which alternates layers of translucent material of different refractive indices so as to cause selective interference of wavelengths with the exception of the wavelength of the color that is to be enhanced. For example, if blue is to be enhanced by 10 percent, the dichroic filter interferes with, thus reducing, the remaining wavelengths of the visible spectrum by an approximate 10 percent factor leaving the blue wavelength transmitted intact with the color blue enhanced by its increased presence relative to the total wavelength presence of the remaining light beam.
Dichroic filters because of their reflective capability are much cooler and have fewer scattering losses than absorptive filters with the result that the dichroic filters have a 5-10% output advantage over absorptive filters.
A problem of filtering devices in general is that the degree of color saturation of the light beam varies across the aperture of the beam primarily because the filter is moved from one side of the beam aperture to the other side and is also positioned in place so that the color saturation varies across the beam aperture and likewise at the beam termination. This undesirable effect is especially noticeable during the change from one color or color saturation level to another.